Straight Dexron ATF or a 50/50 mix of Dexron/kerosene makes a fantastic rust penetrant. I use dexron more often than I use the rust penetrant issued to us at work. I've use it to remove rusty axle nuts that I couldn't remove with either my impact wrench, or my breaker bar and a 5 foot long cheater pipe.
It also works great on rusty forklift chains. Brush or drizzle it on and a week later, wipe it off with a rag. You're left with nice gray steel. I have Kroil and love it, but it's too expensive for everyday use.